Question: Which of these numbers is prime? ${25,\ 27,\ 47,\ 92,\ 99}$
Answer: A prime number has exactly two factors: 1 and itself. The factors of 25 are 1, 5, and 25. The factors of 27 are 1, 3, 9, and 27. The factors of 47 are 1 and 47. The factors of 92 are 1, 2, 4, 23, 46, and 92. The factors of 99 are 1, 3, 9, 11, 33, and 99. Thus, 47 is a prime number.